This invention relates to a mass flow rate measurement circuit of a Coriolis mass flow/density meter.
Coriolis mass flow/density meters, as is well known, have at least one bent or straight flow tube that is vibrated while a fluid flows through it; details are given below in connection with the description of FIG. 1.
Usually, at least one vibrator and at least two vibration sensors are mounted on the flow tube, the vibration sensors being positioned at a given distance from each other in the direction of flow. The flow tube generally vibrates at a mechanical resonance frequency that is predetermined by its material and dimensions but is varied by the density of the fluid. In other cases, the vibration frequency of the flow tube is not exactly the mechanical resonance frequency of the flow tube, but a frequency in the neighborhood thereof.
The vibration sensors deliver sinusoidal or pulse signals whose frequency is equal to the vibration frequency of the flow tube, and which are separated in time, i.e., between which a phase difference exists when the fluid flows through the flow tube. From this phase difference, a time-difference signal, e.g., a signal representing the time difference between edges of the pulsed sensor signals or between zero crossings of the sinusoidal sensor signals, can be derived which is directly proportional to mass flow rate.
U.S. Pat. No. 4,911,006 discloses a mass flow rate measurement circuit of a Coriolis mass flow/density meter comprising a mass flow sensor having two parallel, U-shaped flow tubes through which flows a fluid to be measured and which
vibrate in operation at a frequency determined by their material and dimensions but varied by the density of the fluid, said frequency being equal to or in the neighborhood of the instantaneous mechanical resonance frequency of the flow tubes, and
which have attached to them a first and a second electromagnetic vibration sensor positioned at a given distance from each other in the direction of flow, which deliver a sinusoidal first and a sinusoidal second sensor signal,
as well as a vibrator,
said measurement circuit comprising:
an intermediate switch having a first input fed by the first sensor signal and a second input fed by the second sensor signal;
a first and a second buffer fed, respectively, by the first and second outputs of the intermediate switch and each having an output;
a first and a second zero-crossing detector connected at their input ends to the outputs of the first and second buffers, respectively, and each having an output;
a first-in-time detector connected at its input end to the outputs of the first and second zero-crossing detectors;
an EXOR gate connected at its input end to the outputs of the first and second zero-crossing detectors and having an output;
a 50-MHz oscillator having an output;
an AND gate having
a first input connected to the output of the oscillator, and
a second input connected to the output of the EXOR gate;
a counter having a count output and a pulse input which is connected to the output of the AND gate; and
a microprocessor which generates from the count a signal representative of mass flow rate.
This prior-art measurement circuit is suitable virtually only for mass flow sensors with the above-mentioned U-shaped flow tubes, which vibrate at about 50 Hz to 100 Hz, as is also shown by the above-mentioned 50-MHz oscillator. The period of oscillation of such an oscillator is 20 ns; this is used as a fundamental unit to measure the time difference; therefore, the resolution of this measurement is also 20 ns. This is sufficient for time differences occurring at 50 Hz to 100 Hz.
For mass flow sensors with flow tubes vibrating at higher frequencies, particularly with straight flow tubes, which vibrate at 800 Hz to 1500 Hz, this prior-art measurement circuit is unsuitable. The person of average skill in the art could think of simply increasing the frequency of the 50-MHz oscillator, but this would lead to a frequency of the order of 1 GHz. The implementation of such oscillators would require a superhigh-frequency circuit technology that is not compatible with the low-frequency circuit technology necessary for the remainder of the circuit.
It is therefore an object of the invention to provide a mass flow rate measurement circuit of a Coriolis mass flow/density meter which is also suitable for mass flow sensors employing flow tubes that vibrate at a frequency of the order 1 kHz. Furthermore, this measurement circuit is to be simpler with respect to the necessary circuit technology, particularly as far as the above-mentioned first-in-time detector is concerned.
To attain the object, the invention provides a mass flow rate measurement circuit of a Coriolis mass flow/density meter comprising a mass flow sensor having at least one flow tube through which flows a fluid to be measured and which
vibrates in operation at a frequency determined by its material and dimensions but varied by the density of the fluid, said frequency being equal to or in the neighborhood of the instantaneous mechanical resonance frequency of the flow tube,
has attached to it a first and a second electromagnetic vibration sensor positioned at a given distance from each other in the direction of flow
which deliver a sinusoidal first and a sinusoidal second sensor signal, respectively,
as well as a vibrator, and
is surrounded by a support frame or support tube,
said measurement circuit comprising:
a first and a second impedance-matching device fed by the first and second sensor signals, respectively, and having a very high input resistance, a low output resistance, and an output;
an intermediate switch having a first and a second input connected to the outputs of the first and second impedance-matching devices, respectively;
a third and a fourth impedance-matching device fed, respectively, by a first and a second output of the intermediate switch and having a very high input resistance, a low output resistance, and an output;
a first and a second low-pass filter connected at their input ends to the outputs of the third and fourth impedance matching devices, respectively, and having an output, a passband, and an upper cutoff frequency, with
the upper cutoff frequency of the first low-pass filter differing by about 10% to 15% from the upper cutoff frequency of the second low-pass filter, and
the passband covering at least the vibration frequency values occurring in operation;
a first and a second zero-crossing detector fed by the outputs of the first and second low-pass filters, respectively, and each having an output;
a time-to-digital converter having a start input, a stop input, and a clock input and delivering a digital signal,
the start input being connected to the output of the first zero-crossing detector, and
the stop input being connected to the output of the second zero-crossing detector;
a high-frequency clock generator having an output coupled to the clock input of the time-to-digital converter; and
a microprocessor which generates a signal representative of mass flow rate from the digital signal and a signal representative of a calibration factor and controls the switching of the intermediate switch.
One advantage of the invention is that the above-mentioned EXOR gate and the above-mentioned first-in-time detector are replaced by the two bandpass filters. Another advantage is that the problem associated with the increase of the frequency of the above-mentioned 50-MHz oscillator to a frequency of the order of 1 GHz is circumvented by the use of a time-to-digital converter. If clocked at 50 MHz, for example, this time-to-digital converter has a time resolution of typically 100 ps.